EM Induction

Question 1

What are the 2 ways of generating electricity

Answer 1

Using an…
a) Alternator
b) Dynamo

Question 2

Do alternators produce AC or DC

Answer 2

AC

Question 3

What does an alternator consist of

Answer 3

A coil of wire rotating in a magnetic field created by a permanent magnet

Question 4

Describe structure of alternator

Answer 4

Current flows in from external circuit
As the coil is rotated in the magnetic field a p.d is induced in one direction
As the rotation continues the p.d reverses direction
Slip rings are in constant contact with the same side of the coil
Carbon brushes make continuous contact with external circuit and slip rings

Question 5

How can the maximum p.d or current in an both an alternator and a dynamo be increased?

Answer 5

Increasing rate of rotation
Increasing strength of magnetic field
Increasing number of turns on the coil

Question 6

Does a dynamo produce ac or dc current

Answer 6

DC

Question 7

What does a dynamo consist of

Answer 7

A coil of wire rotating in a magnetic field as an alternator does
HOWEVER it uses a split ring commutator rather than 2 slip rings

Question 8

What happens in a dynamo

Answer 8

A split ring commutator changes the coil connections every half turn.
As the induced potential difference is about to change direction, the connections are reversed.
This means that the current to the external circuit always flows in the same direction.

Question 9

What happens in a moving coil microphone

Answer 9

Pressure variations in sound waves are converted into a varying electric current
1. Pressure variations cause flexible diaphragm to vibrate
2. Diaphragm is connected to a coil of wire which is wrapped around a permanent magnet
3. The movement of the coil produces a current in the coil via electromagnetic induction

Question 10

How does a loudspeaker work

Answer 10

A loudspeaker converts variations in an electric current into sound waves
1. A coil of wire inside the loudspeaker is wrapped around a permanent magnet
2. When a current flows through the coil there is a force that makes the diaphragm moves
3. Changes in the current causes the movement of the diaphragm which produces the sound waves

Question 11

What is a transformer?

Answer 11

A device that can change the p.d of an alternating current

Question 12

Does a step up transformer increase or decrease the p.d

Answer 12

Increase

Question 13

Does a step down transformer increase or decrease the p.d

Answer 13

Decrease

Question 14

What happens when a transformer is working

Answer 14

A primary pd drives an AC current through a primary coil
The primary coil produces a magnetic field which changes as current changes
Iron core increases the strength of the magnetic field
The changing magnetic field induces a changing pd in the secondary coil
The induced potential difference produces an AC current in the external circuit

Question 15

What is another name for electromagnetic induction

Answer 15

The generator effect

Question 16

Describe the structure of a transformer

Answer 16

A transformer is made from two coils of wire, a primary coil from the (ac) input and a secondary coil leading to the ac output.
The coils are not electrically connected.
Instead, they are wound around an iron core.
This is easily magnetised and can carry magnetic fields from the primary coil to the secondary coil.

Question 17

What is the transformers equation

Answer 17

Vp Np
—- = —-
Vs. Ns

Question 18

Equation for power in transformers

Answer 18

Ip x Vp = Is x Vs

Question 19

What do the symbols used in the equations for transformers mean

Answer 19

N= number of coils
V= voltage
I = current
p = primary
s = secondary

Question 20

Why do the national grid transmission lines use high voltages

Answer 20

Power = Voltage x current means that for a given power transfer, the higher the voltage used, the lower the current needed.

Heat energy wastage through electrical resistance is proportional to the square of the current, as given by the equation Power = Current^2 x resistance. Reducing the current can create huge reductions in energy lost to the surroundings through resistance.

Question 21

Why are transformers used in the national grid

Answer 21

To ensure the transmission of electricity is efficient
They increase voltage in order to decrease current
Decreasing current means less energy is lost as heat
Voltage is then decrease for safer sue in our homes and industry